Method of treating magnesium



Patented Sept. 15, 1942 2,295,843 Me'rnon or 'ramrmo MAGNESIUM .Joseph D. Hanawalt and Charles E. Nelson, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich a corporation of Michigan No Drawing. Application November 16, 1940, Serial No. 365,987

6 Claims.

This invention relates to methods of producing coatings upon magnesium and alloys thereof. It more particularly concerns an improved methd of treating an article of magnesium or an alloy thereof, in which magnesium is the predominant constituent, having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day to produce thereon a decorative, strongly adherent coating in a wide V variety of colors.

Corrosion rates referred to herein and in the appended claims are determined by immersing a specimen of magnesium or alloy thereof for one minute in a 3 per cent aqueous sodium chloride solution held at 30 C. and then exposing the specimen to air for two minutes and repeating these steps in succession for several months.

The average loss in weight of the specimen, expressed in milligrams per square centimeter per day, after being subjected to the above treatment for from 1 to 4 months, is used as ameasure of corrodibility.

v Magnesium and magnesium alloys in which the impurities iron, nickel, copper, and silicon are present in relatively small amounts, the limiting amount of each depending upon the particular composition, have corrosion rates of not substantially more than 0.2 milligram per square centimeter per day. We have made a great number or magnesium alloys having such cor rosion characteristics, the following being illustrative examples of compositions having a corrosion rate of not substantially more than 0.2

' milligram per square centimeter per day: (1) a to 12.0 per cent aluminum, and from .01 to 2.0

per cent manganese, and having .001 per cent-of the impurity iron present, .0005 per cent of the impurity nickel present, and .03 per cent eachof the impurities copper and silicon present, (3) a magnesium base alloy containing as the alloying ingredients from 2.0 to 8.0 .per cent aluminum, .01 to 2.0 per cent manganese, 1.0 per centor less of zinc and having .001 per cent each of the impurities iron and nickel present, and .03

per cent of each of the impurities copper and silicon present, (4) a magnesium base alloy containing as the alloying ingredients from 2.0 to

8.0 per cent aluminum, .01 to 2.0 per cent manganese, 3.0 per cent or more of zinc and having .002 per cent of the impurity iron present, .001 per cent of the impurity nickel present, and .1 per cent of the impurities copper and silicon present, (5) magnesium having .002 per cent of the impurity'iron present, .0002 per cent of the impurity nickel present, .03 per cent of the impurity copper present, and .02 per cent of the impurity silicon present. The amounts of the impurities present inthe illustrative compositions listed above are not to be construed as limiting amounts, since lesser or somewhat greater amounts may be present. Alloys such as those described above may be made in several ways, as, for example, by melting pure distilled magnesium in a crucible of graphite of high purity under a flux consisting of per cent magnesium chloride and 30 per cent sodium chloride and adding the pure alloying ingredient or ingredients thereto.

The limiting amount of the impurity or im-. purities, as the case may be, that may be present in order to give analloy that has a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day depends not only upon the particular composition or alloy but upon the number of the-impurities that are present. While the corrosion rate above specified is very small compared to that of the known commercial magnesium or its alloys, once the limiting amount of impurity or impurities, as the case may be, is exceeded by only a small amount, ingeneral the corrosion rate at once becomes that of ordinary commercial magnesium or its alloys.

While magnesium and alloys thereof in which the impurities are present in relatively small amounts are already very resistant to corrosion in comparison with commercial magnesium and its alloys, we have discovered that we can apply a coating thereto which is of a highly decorative,

nature, and, in addition, the coating renders the magnesium article or alloy thereof even more highly resistant to corrosion.

The invention, then, consists in the method a thick coating is not produced, but instead a film-like coating is formed which is very adherent and offers a high degree of protection particularly under atmospheric conditions. In the case of the pure magnesium metal a coating of practically any desired thickness can be produced also characterized by being hard and dense. Further, we have discovered that the coating so produced may be dyed a wide variety of colors, or, if desired, the dye may be included in the bath prior to applying the coating, in which case a dyed coating is produced.

While magnesium or its alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day are readily coated in accordance with the present invention, commercial magnesium and its alloys having higher rates of corrosion are not protectively I coated when subjected to treatment according to our discovery, but instead are corroded. For example, if an article of magnesium or its :alloy having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day is immersed in a bath of boiling water, a very dense, hard glossy, adherent coating is produced on the article in a short time, and continued action of the bath serves to increase the thickness of the coating. subjecting an article of magnesium such as was previously known in the art to an identical treatment causes hydrogen to be liberated from the water and magnesium hydroxide to form at the surface of the article. The magnesium hydroxide formed does not adhere firmly and uniformly to the surface so as to form a continuous. protective coating, but sloughs away from the article in places so as to leave a nonuniform pitted coating.

While it is known that commercial magnesium could be coated by treatment with water at temperatures above 100 -C. and at pressures well above atmospheric in the manner taught by Backer, U. S. Re. 16,340, it was not previously known that hard, dense, glossy coatings capable of being dyed could be produced on any article of magnesium by subjecting it to the action of hot water at a temperature of 100 C. or below at atmospheric pressures.

In carrying out the invention, the magnesium or magnesium alloy article to be coated is first cleaned, as by grinding, acid pickling, or bufling, according to the degree of luster and smoothness desired in the finished coating. The degree of luster and smoothness of the finished article depends upon the degree of smoothness of the article before it is subjected to the action of the coating bath. The article to be coated, having been cleaned, is then subjected to the action of hot water. The coating produced is hard and highly resistant to corrosion, particularly in contact with dissimilar metals, besides being highly decorative in nature.

The temperature of the coating bath may .be held between about 85 to 100 C., and preferably at about 100 C. It is also preferable to operate the bath at atmospheric pressure. The article being coated should be subjected to the action of the bath for between about to 30 hours or more, depending upon the thickness of the coating desired and the temperature of the bath. At 100 C. a satisfactory coating can be produced on pure magnesium in 10 hours.

The term water referred to herein and inthe appended claims is intended to include not only distilled water but potable waters and ground waters ordinarily available from wells, springs,

The coating so produced may be readily dyed by subjecting the coated article to the action'oi anaqueous solution of a suitable dye at a temperature between about and C. for about /2 hour or more. Pure magnesium metal coated in the above described manner has been found to be particularly amenable to a dyeing treatment. Suitable dyes to employ are those not discharged by an alkali and not decomposed at the temperature employed, and soluble in water. The term not discharged by an alkali" used herein and in the appended claims means that the presence of an alkali neither prevents the dye from coloring the article nor affects the fastness of the color produced by the dye. Examples of such dyes are Alizarin Cyanine, Janus Green, Safranine, Gentian Violet, and the like. The amount of dye to employ should be between about 0.1 and 0.5 per cent or more by weight of the dyeing solution, depending upon the shade desired. In general, the more concentrated the dye in the solution, the

deeper will be the shade of color produced. If

desired, a dye having the characteristics aforementioned may be added to the coating bath, in which case as the coating is produced it is dyed or colored the desired shade.

By the foregoing method coatings that are hard, highly resistant to corrosion, and strongly adherent may be produced having practically any desired shade of color.

The present application is a continuation-inpart of application Serial No. 253,343, filed January 28, 1939.

We claim:

1. In a methor of producing a coating on an article of magnesium and its. alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day, determined by measuring the average loss in weight of the magnesium specimen alternately immersed for 1 minute in a 3 per cent sodium chloride solution held at 30 C. and exposed to air for 2 minutes for an extended period of time, the step which com rises subjecting the article to the action of a liquid consisting entirely of water at a temperature of between about 85-100 C. at normal atmospheric pressure.

2. The method of treating an article of magnesium and its alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day to produce a coating thereon. comprising subjecting the article to the action of a liquid consisting entirely of water at a temperature between about 85-100 C. at atmospheric pressure for from to 30 hours. v

3, The meth'od of producing a colored coating on a light metal article comprising subjecting an article of magnesium and its alloys having a corrosion rate of not substantiallymore than 0.2 milligram per square centimeter per day to the action of a liquid consisting entirely of water at 4. The method of coating and dyeing a light I metal article comprising subjecting an article of magnesium and its alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day to the action of a liquid consisting entirely of water at a tempera ture between about 85 and 100 C. at atmospheric pressure, whereby a coating is produced on said article and dyeing the coated article by immersing it in an aqueous solution of a dye not discharged by an alkali.

5. The method of producing a colored coating on a relatively pure magnesium article having a corrosion rate of not, substantially more than 0.2 milligram per square centimeter per day, the step which comprises subjecting said article to the action of a liquid consisting entirely of water at a, temperature between about 85 and 100 C. at atmospheric pressure, said water containing a dye soluble therein and not discharged by an alkali, wherebya colored coating is produced on said article.

6. The meth'od of coating and dyeing a relatively pure magnesium article having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day, the steps which comprise subjecting said article to-the action of a liquid consisting entirely of water at a temperature between about 85 and 100 C. at atmospheric pressure whereby a coating is produced on said article, and thereafter dyeing the coated article by immersing it in an aqueous solution of a dye not discharged by an alkali.

JOSEPH D. HANAWALT. CHARLES E. NELSON. 

